1. Technical Field
The present invention relates to publish-subscribe systems, and more particularly, to a method and system for semantic publish-subscribe services.
2. Discussion of the Related Art
Publish-subscribe systems interconnect information providers to information consumers in a distributed environment. In such systems, information providers publish information in the form of events, information consumers subscribe to events that satisfy certain conditions specified by the consumers and the system ensures the timely delivery of published events to all interested subscribers.
Publish-subscribe systems are often classified as topic-based (or group-based) and content-based. In topic-based systems such as those described in B. Oki, M Pfluegl, A. Siegal, and D. Skeen. “The Information Bus: An Architecture for Extensible Distributed Systems”. Operating Systems Review, 27(5):58-68, December 1993 and D. Powell. “Group Communications”. Communications of the ACM, 39(4):50-97, April 1996, publishers label each event with a topic name, while consumers subscribe to all events in a particular topic.
In content-based systems such as those described in G. Cugola, E. Nitto, and A. Fuggetta. “Exploiting an Event-based Infrastructure to Develop Complex: Distributed Systems” Proc. IEEE International Conference on Software Engineering (ICSE), pp. 261-270, 1998, M. Aguilera, R. Strom, D. Sturman, M. Astley. and T. Chandra. “Matching Events in a Content-based Subscription System”, Proc. ACM Symposium on Principles of Distributed Computing (PODC), pp. 53-61, 1999 and A. Carzaniga, D. Rosenblum, and A. Wolf. “Design and Evaluation of a Wide-Area Event Notification Service”. ACM Transactions on Computer Systems, 19(3):332-383, August 2000, subscribers can choose filtering criteria along multiple dimensions, and they get all events that meet the specified criteria.
These filtering criteria are often expressed using attribute-value pairs or extensible markup language (XML)-based tree structures. A drawback of such topic-based or content-based publish-subscribe systems is the lack of expressivity when users specify their subscription interests or label their published events.
In recent years, a few publish-subscribe systems have been proposed to match subscriptions and events based on their semantics. The S-ToPSS system described in M. Petrovic, I. Burcea, and H. Jacobsen. “S-ToPSS: Semantic Toronto Publish/Subscribe System”. Proc. International Conference on Very Large Data Bases (VLDB), pp. 1101-1104, 2003, takes into account synonyms and concept hierarchies in the matching process. The Ontology-based Pub-sub System (OPS) described in J. Wang, B. Jin, and J. Li. “An Ontology-based Publish/Subscribe System”. Proc ACM/IFIP/USENIX 8th International Middleware Conference, pp. 232-253, 2004, uses resource description framework (RDF) graphs and graph patterns to represent subscriptions and events. P. Chirita, S. Idreos, M. Koubarakis, and W. Nejdl. “Publish/Subscribe for RDF-based P2P Networks”. Proc. European Semantic Web Symposium, 2004 uses RDF to enhance publish-subscribe services in peer to peer (P2P) networks. While these systems are more expressive than the traditional topic-based or content-based systems, they still suffer from several drawbacks.
For example, they do not make use of reasoning or domain knowledge in ontologies to aid the matching process, thus they may fail to identify semantically relevant events for the subscribers. In addition, they typically use restricted graphs (e.g., all RDF triples in an event must have the same subject) to represent subscriptions and events, thus they are less expressive than an ideal system based on general graphs. Further, they all have scalability limits when the size of the system increases, and generally, do not work well under a large number of subscriptions and a high volume of events.